The present invention pertains to a sputtering target having zinc sulfide as its principal component which allows direct current (DC) sputtering, which has minimal arcing during the sputtering, and which is capable of reducing particles (dust emission) and nodules upon forming a film with sputtering, and which is of high density and having minimal variation in quality and capable of improving mass productiveness, as well as to an optical recording medium having formed thereon a phase change optical disc protective film having zinc sulfide as its principal component employing this target, and the manufacturing method of such a sputtering target.
In recent years, high-density recordable optical disc technology capable of recording/reproduction without requiring a magnetic head has been developed, and is rapidly attracting attention. This optical disc can be classified into the three categories of reproduction-only, recordable and rewritable. Particularly, the phase change method employed in the recordable and rewritable type discs is attracting attention. The fundamental principle of the recording/reproduction employing this phase change optical disc is briefly described below.
This phase change optical disc performs the recording/reproduction of information by heating and increasing the temperature of a recording thin film on a substrate by irradiating a laser beam thereto, and generating a crystallographic phase change (amorphous crystal) in the structure of such recording thin film. More specifically, the reproduction of information is conducted by detecting the change in the reflectivity caused by the change in the optical constant of the phase.
The aforementioned phase change is performed with the irradiation of a laser beam narrowed down to a diameter of approximately 1 to several μm. Here, for example, when a 1 μm laser beam passes through at a linear velocity of 10 m/s, light is irradiated to a certain point on the optical disc for 100 ns, and it is necessary to perform the aforementioned phase change and detect the reflectivity within such time frame.
Moreover, in order to realize the foregoing crystallographic phase change; that is, the phase change of the amorphous and crystal, not only will the phase change recording layer be subject to fusion and quenching more than once, the peripheral dielectric protective layer and aluminum alloy will also be repeatedly subject thereto.
In light of the above, a phase change optical disc has a four-layer structure wherein, for instance, both sides of a Ge—Sb—Te recording thin film layer are sandwiched with a zinc sulfide-silicic oxide (ZnS—SiO2) high-melting point dielectric, and an aluminum alloy reflective layer is additionally provided thereto.
Among the above, in addition to being demanded of an optical function capable of increasing the absorption of the amorphous portion and crystal portion and which has a large reflectivity difference, the reflective layer and protective layer are also demanded of a function for preventing the deformation caused by the moisture resistance or heat of the recording thin film as well as a function for controlling the thermal conditions upon recording (c.f. “Kogaku” magazine, volume 26, no. 1, pages 9 to 15).
As described above, the protective layer of a high-melting point dielectric must be durable against repeated thermal stress caused by the heating and cooling, must not allow such thermal effect to influence the reflective film or other areas, and it is also required to be thin, of low reflectivity, and possess strength to prevent deterioration. From this perspective, the dielectric protective layer plays an important role.
The dielectric protective layer described above is usually formed with the sputtering method. This sputtering method makes a positive electrode target and a negative electrode target face each other, and generates an electric field by applying a high voltage between the substrates thereof and the targets under an inert gas atmosphere. The sputtering method employs a fundamental principle where plasma is formed pursuant to the collision of electrons, which are ionized at such time, and the inert gas, the positive ion in this plasma extrudes the target structured atoms by colliding with the target (negative electrode) surface, the extruded atoms adhere to the opposing substrate surface and the film is formed.
Conventionally, since the aforementioned protective layer is demanded of permeability, heat resistance, and so on in a visible optical band, sputtering is conducted with a ceramic target such as ZnS—SiO2 in order to form a thin film of approximately 500 to 2000 Å. Nevertheless, with these materials, since the bulk resistance of the target is high, deposition cannot be performed with a direct current sputtering device, and, usually, a high frequency sputtering (RF) device is used.
However, not only is this high frequency sputtering (RF) device expensive as a device, it also has numerous problems in that the sputtering efficiency is inferior, power consumption is considerable, control is difficult, and deposition speed is slow. Moreover, in order to increase the deposition speed, when high power is applied thereto, the substrate temperature will rise, and there is a problem in that the polycarbonate substrate will deform.
Further, with respect to the SiO2 employed in the foregoing zinc sulfide-silicic oxide (ZnS—SiO2) target, SiO2 having an average grain size of 0.1 to 20 μm at a purity level of 4N or more is ordinarily used, and the target is manufactured by being sintered at 700 to 1200° C.
The target containing SiO2 in ZnS often generates arcing upon forming a film with sputtering, and particles (dust emission) and nodules arise during sputtering as a result thereof, and, not only does this reduce the evenness and quality of the deposition, there is also a problem in that the productivity will also deteriorate.